1. Field of the Invention
The present invention relates to an igniting apparatus of a discharge lamp, and more specifically, to a discharge lamp igniting apparatus for performing a feedback control so as to control light of a fluorescent lamp and the like. The present invention also relates to a discharge lamp igniting apparatus provided with a function for correctly judging a final lifetime stage of a discharge lamp.
2. Description of the Related Art
A dimming control apparatus for controlling light of a discharge lamp and the like is known in the art. To prevent either flickers or moving flanges which occur when the light level is deep (dark) and the air temperature is low, the technique related to the feedback control is known from, for example, Japanese Laid-open Patent Application No. 6-302393. FIG. 1 is a structural diagram for showing an outline of this known technique.
In this example, the power supplied from the high frequency power supply 2 to the discharge lamp 4 of the load circuit 3 is detected by the current detecting circuit 5. Based upon the lamp current signal "A" detected by this detecting circuit 5 and the externally inputted dimming control signal "B", the control signal E for controlling the high frequency power supply 2 is produced by the feedback control circuit 7. The feedback control circuit 7 subtracts the lamp current signal A from the dimming control signal B, and the error signal "C" is amplified by the error amplifying circuit 9. Furthermore, the amplified signal "D" is added to the dimming control signal B to thereby produce the control signal E. FIG. 2 indicates a relationship between the dimming control signal B and the lamp current signal A under room temperature.
In this example, when the value of the lamp current signal A is the same as the dimming control signal B, the error signal C becomes "0", and also the output from the error amplifying circuit 9 becomes "0". Accordingly, the dimming control signal B is directly entered as the control signal E into the high frequency power supply 2. When the value of the lamp current signal A is lower than the dimming control signal B, the error signal C is produced, and then is amplified by the error amplifying circuit 9. Furthermore, the amplified error signal is added to the dimming control signal B, so that the control signal E is formed, and is entered into the higher frequency power supply 2. As described above, the feedback control is carried out in response to the dimming control signal B, so that the lamp current signal A is increased so as to prevent flickers, or moving flanges of the discharge lamp 4.
In this sort of discharge lamp igniting apparatus, since the feedback control circuit 7 into which the lamp current signal A is inputted is operated by the low voltage power supply under normal condition, when the detected lamp current signal A becomes high, the capability of the feedback control circuit 7 must be increased. However, as indicated in FIG. 14, in the above-described case, the lamp current signal A is detected which should be controlled with respect to the entire range of the dimming control signal B. Even when the light level is low (shallow) and thus the necessity of the feedback control is lowered, the feedback control becomes active.
In other words, most of the cases that the feedback control is required are such cases that the light level is deep (dark). This corresponds to only 1/3 to 1/5 of the range for detecting the lamp current signal A with respect to the overall dimming control range of the dimming control signal B. In other cases, waste feedback controls are carried out. As a consequence, in the above-described example, the lamp current signal A is carried out in a waste manner with respect to the overall dimming control range of the dimming control signal B. As a result, there are problems that the dynamic range of the actually required detecting range is relatively narrowed, and the detection precision would be decreased, and also the noise resistibility would be deteriorated.
FIG. 3 is a structural diagram for explaining another conventional discharge lamp igniting apparatus described in, for instance, Japanese Laid-open Patent Application No. 64-3995. In the drawing, reference numeral 101 indicates the commercial power supply, reference numeral 102 denotes the high frequency power supply for converting the DC power converted by the rectifying/smoothing circuit 124 into the high frequency power, and reference numeral 103 indicates the load circuit containing the discharge lamp 104. Reference numeral 105 shows the voltage detecting circuit for detecting the voltage of the discharge lamp 104, reference numeral 106 indicates the detection voltage detected by the voltage detecting circuit 105, and reference numeral 110 represents the control circuit in which the comparator 107 compares the detection voltage 106 with the first reference voltage 108a, and the control signal 112 is sent to the high frequency power supply 102.
Next, a description will be made of operations of the discharge lamp igniting apparatus with reference to FIG. 3. When the discharge lamp 104 reaches the final lifetime stage, the half wave discharge will occur in which a discharge occurs only from one electrode of the discharge lamp 104, and no discharge occurs from the other electrode thereof. When the half wave discharge occurs, since the impedance of the discharge lamp 104 is increased, a high voltage is produced from the voltage detecting circuit, as compared with that when the normal discharge occurs. The detection voltage 106 detected by the voltage detecting circuit 105 is compared with the first reference voltage 108a by the comparator 107 of the control circuit 110, and then when the detection voltage 106 is higher than the first reference voltage, the control circuit judges that the discharge lamp is in the final lifetime stage. Then, the control circuit 110 sends the control signal for interrupting, or reducing the output of the high frequency power supply 102 in order to prevent the discharge lamp igniting apparatus from being destroyed.
Now, the operation characteristic while the discharge lamp 104 is turned ON is shown in FIG. 4. The discharge lamp 104 under turn-ON state owns the negative characteristic that the discharge voltage is gradually increased in connection with the decrease of the discharge current, as shown in FIG. 4. As a consequence, since the impedance of the discharge lamp 104 in the final lifetime stage is large, the discharge current becomes low. As a result, the voltage of the discharge lamp in the final lifetime stage is higher than that under normal turn-ON state.
Since the conventional discharge lamp igniting apparatus is arranged in the above-described manner, in the discharge lamp igniting apparatus having no such a dimming control function for variably controlling the light flux emitted from the discharge lamp, either the voltage of the discharge lamp during the normal operation or the voltage of the discharge lamp during the final lifetime stage is produced from the voltage detecting circuit. As a consequence, in order to correctly judge the final lifetime stage of the discharge lamp, the first reference voltage to be set into the comparator is suitably located in the voltage region shown in FIG. 5.
However, in the discharge lamp igniting apparatus equipped with the dimming control function, as shown in the operation characteristic of FIG. 6, the voltage of the discharge lamp under normal condition is gradually increased in connection with the decrease of the light level with respect to the full light. As a result when the first reference voltage to be set to the comparator is set to, for example, a zone "A" of FIG. 6, namely, when the level of the first reference voltage is set to high, it could not correctly judge the final lifetime stage of the discharge lamp near the full light (light level: 10%). Thus, there is a risk that the discharge lamp igniting apparatus would be destroyed. Also, when the first reference voltage to be set to the comparator is set to a zone "B" of FIG. 6, namely when the level of the first reference voltage is set to low, there is a problem. That is, the discharge lamp operated under normal condition in the low dimming control range is erroneously judged as the discharge lamp operated in the final lifetime stage. As described above, it is very difficult to set the range of the first reference voltage.